Nitrile rubber has been in commercial use for over 40 years. It is generally prepared by copolymerizing acrylonitrile with butadiene in presence of an emulsifier, a free radical catalyst and activator, as well as a chain-transfer agent. Polymerization can be carried out at temperatures below 0.degree. C. to above 80.degree. C. in absence of oxygen. Blends of linear nitrile rubber polymers with precrosslinked nitrile rubber polymers are also known. The precrosslinked polymers are prepared in presence of a crosslinking monomer, such as divinyl benzene, which imparts improvements to certain physical properties but sacrifices others.
Precrosslinked nitrile rubber polymers, obtained by addition of divinyl benzene to a polymerization recipe, are disclosed in the prior art. In this connection, please note the text on pp. 85 and 135 in Rubber Chemistry Technology, Vol. 36, No. 5, for December, 1963, and Vol. 37, No. 2, for April-June, 1964, published by Division of Rubber Chemistry of the ACS. The text discloses that crosslinked nitrile rubbers with high gel content were blended with other nitrile rubbers to improve extrusion, calendaring and dimensional stability during curing. However, the text also points out that due to a basic change in the molecular structure, properties of polymers were affected by crosslinking. These polymers not only lost much of their plasticity but also had different properties after curing. Tensile strength and ultimate elongation were much reduced; snappiness and hardness increased; rebound resilience, resistance to tear and low temperature behaviour deteriorated while compression set and swelling properties were improved. Although the novel materials of this invention show improvements in essentially the same properties as the prior art materials, they are not accompanied by the expected sacrifice of other physical and processing properties.
Another piece of prior art which is pertinent to the subject matter herein is the 1968 article by S. L. Rosen entitled "Some Rheological Properties of a Linear-Gel Polymer System" which appeared in Applied Polymer Symposia, No. 7, pp. 127-141. This article acknowledges the known practice of blending crosslinked polymers with linear polymers to improve melt processing of the blends and reports experiments with blends of crosslinked and linear polyethylacrylate latices prepared by emulsion polymerization with 3,10 and 30% triethylene glycol dimethacrylate as the crosslinking monomer. It should be apparent to one skilled in the art that the polyethylacrylate system is vastly different from the nitrile rubber system in that the polyethylacrylate system, as disclosed in the article, cannot be vulcanized and, therefore, is totally useless for any of the conventional rubber applications.